Observation of plasma deceleration at a rotational magnetopause discontinuity

Aggson, T. L.; Maynard, N. C.; Ogilvie, K. W.; Scudder, J. D.; Gambardella, P. J.

doi:10.1029/gl011i001p00008

Cited by 22 publications

(9 citation statements)

References 14 publications

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“…In recent years, plasma, magnetic, and electric [1981]) events by using electric and magnetic field as well as plasma data. Using this same technique, Aggson et al [1984] have reported a rotational magnetopause traversal in which the observed fluid speed decreased; they pointed out that this behavior was permitted for the geometrical circumstances encountered under the MHD description of RD's as theoretically outlined, for example, by de Hoffman and Teller [1950] and mentioned by Roedeter [1977] in connection with the magnetotail magnetopause and by Crooker [1979] in connection with specialized cusp geometries. Cowley et al [1983] have argued that certain reverse draping observations may be in this same category.…”

Section: Introductionmentioning

confidence: 82%

Fluid signatures of rotational discontinuities at the Earth's magnetopause

Scudder¹

1984

J. Geophys. Res.

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Fluid signatures in the MHD approximation at rotational discontinuities (RD) of finite width called rotational shear layers (RSL) are examined for general flow and magnetic geometries. Analytical and geometrical arguments illustrate that the fluid speed can either go up or down across an RSL for a fixed normal mass flux. The speed profile may or may not be monotonic with the phase of the magnetic rotation depending on the boundary conditions. The flow velocity may or may not be field aligned or “jetting” as a result of traversing the RSL. In general, significant “convection” is expected in the layer. The observable signatures of (MHD) RSL's depend on seven (boundary condition) parameters: (1) the mass density, (2)–(5) the incident normal and transverse components of the magnetic field and fluid velocity, (6) the angle ε between the incident tangential flow velocity and tangential magnetic field, and (7) the size of the magnetic angular rotation implemented by the layer Δϕ. This general survey illustrates the singular character of the Petschek geometry and that the model predictions of jetting and monotonic speed increases through the layer may not be used as general signatures of RSL's. This is especially true along the Levy, Petschek, and Siscoe curved magnetopause boundary, where the fluid flow, Δϕ, and ε are generally all different from the Petschek regime. Accordingly, the lack of observed speed increases across the magnetopause can no longer be used by itself to infer that the magnetopause is locally closed. Of the spectrum of MHD RSL's that are possible, the “accelerating” ones require special boundary conditions most generally occurring near the nose of the magnetopause; the “decelerating” RSL's become increasingly more probable at magnetopause crossings removed from the subsolar point. Documented RSL's have been located within the spectrum of possible signatures; the number of RSL's reported to date implies only a lower bound for their frequency of occurrence, since surveys conducted to date have excluded the decelerating ones.

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Section: Introductionmentioning

confidence: 82%

Fluid signatures of rotational discontinuities at the Earth's magnetopause

Scudder¹

1984

J. Geophys. Res.

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“…This is a necessary, but not sufficient, condition for merging to have occurred. While accelerated plasmas were observed during some ISEE-1 magnetopause crossings, Aggson et al (1984) also reported decelerated flows. Moreover, they emanated from a merging site poleward of the satellite.…”

Section: Merging Primermentioning

confidence: 89%

Coupling the Solar-Wind/IMF to the Ionosphere Through the High Latitude CUSPS

Maynard¹

2005

Surv Geophys

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Magnetic merging is a primary means for coupling energy from the solar wind into the magnetosphere-ionosphere system. The location and nature of the process remain as open questions. By correlating measurements from diverse locations and using large-scale MHD models to put the measurements in context, it is possible to constrain our interpretations of the global and meso-scale dynamics of magnetic merging. Recent evidence demonstrates that merging often occurs at high latitudes in the vicinity of the cusps. The location is in part controlled by the clock angle in the interplanetary magnetic field (IMF) Y-Z plane. In fact, B Y bifurcates the cusp relative to source regions. The newly opened field lines may couple to the ionosphere at MLT locations of as much as 3 hr away from local noon. On the other side of noon the cusp may be connected to merging sites in the opposite hemisphere. In fact, the small convection cell is generally driven by opposite hemisphere merging. B X controls the timing of the interaction and merging sites in each hemisphere, which may respond to planar features in the IMF at different times. Correlation times are variable and are controlled by the dynamics of the tilt of the interplanetary electric field phase plane. The orientation of the phase plane may change significantly on time scales of tens of minutes. Merging is temporally variable and may be occurring at multiple sites simultaneously. Accelerated electrons from the merging process excite optical signatures at the foot of the newly opened field lines. All-sky photometer observations of 557.7 nm emissions in the cusp region provide a ''television picture'' of the merging process and may be used to infer the temporal and spatial variability of merging, tied to variations in the IMF.

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“…While increased velocities were observed during some ISEE-1 magnetopause crossings, Aggson et al (1984) also reported decelerated flows. Moreover, they emanated from a merging site poleward of the satellite.…”

Section: Introductionmentioning

confidence: 98%

Polar, Cluster and SuperDARN evidence for high-latitude merging during southward IMF: temporal/spatial evolution

Maynard¹,

Ober²,

Burke

et al. 2003

Ann. Geophys.

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Abstract. Magnetic merging on the dayside magnetopause often occurs at high latitudes. Polar measured fluxes of accelerated ions and wave Poynting vectors while skimming the subsolar magnetopause. The measurements indicate that their source was located to the north of the spacecraft, well removed from expected component merging sites. This represents the first use of wave Poynting flux as a merging discriminator at the magnetopause. We argue that wave Poynting vectors, like accelerated particle fluxes and the Walén tests, are necessary, but not sufficient, conditions for identifying merging events. The Polar data are complemented with nearly simultaneous measurements from Cluster in the northern cusp, with correlated observations from the Super-DARN radar, to show that the locations and rates of merging vary. Magnetohydrodynamic (MHD) simulations are used to place the measurements into a global context. The MHD simulations confirm the existence of a high-latitude merging site and suggest that Polar and SuperDARN observed effects are attributable to both exhaust regions of a temporally varying X-line. A survey of 13 merging events places the location at high latitudes whenever the interplanetary magnetic field (IMF) clock angle is less than ∼150 • . While inferred highlatitude merging sites favor the antiparallel merging hypothesis, our data alone cannot exclude the possible existence of a guide field. Merging can even move away from equatorial latitudes when the IMF has a strong southward component. MHD simulations suggest that this happens when the dipole tilt angle increases or when IMF B X increases the effective dipole tilt.

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Observation of plasma deceleration at a rotational magnetopause discontinuity

Cited by 22 publications

References 14 publications

Fluid signatures of rotational discontinuities at the Earth's magnetopause

Fluid signatures of rotational discontinuities at the Earth's magnetopause

Coupling the Solar-Wind/IMF to the Ionosphere Through the High Latitude CUSPS

Polar, Cluster and SuperDARN evidence for high-latitude merging during southward IMF: temporal/spatial evolution

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